1. Field of the Invention
The present invention relates to a low-profile antenna, and more particularly, to an antenna capable of receiving vertical polarized signals with a planar structure rather than an obliquely disposed structure.
2. Discussion of Related Art
For vehicle communication, a vehicle antenna that is reliable and inexpensive and can be simply manufactured is required. The vehicle antenna needs to be mounted at a position where a signal can be efficiently received. Most research on vehicle antennas has been conducted in regard to various mounting positions, such as a window, a wheel, a vehicle body, and a vehicle roof. For example, research has been conducted on a case where a vehicle antenna is mounted for digital terrestrial reception on the top of the front and rear windows, and other research has been conducted on an influence that vehicle equipment has on the performance of an antenna mounted on a window. Also, electromagnetic simulation results of a global positioning system (GPS) antenna mounted on a windshield have been disclosed.
A vehicle roof is a particularly good position to mount an antenna. An antenna installed on a vehicle roof needs to have a low profile to be protected from a severe environment, and the appearance of a vehicle also needs to be considered. For these reasons, a variety of vehicle roof-mounted antennas, such as a monopole antenna, a planar inverted-F antenna (PIFA), and a printed circuit board (PCB) antenna, have been suggested. However, such a protruding antenna is easily damaged by an environmental condition and may ruin the profile of a vehicle. Thus, a low-profile antenna, such as a hidden antenna mounted on a vehicle roof, is required as a roof-mounted antenna.
Due to horizontal polarization, a low-profile antenna is easily designed for satellite communication. On the other hand, it is difficult to implement a low-profile antenna having a characteristic of receiving vertical polarized signal for a terrestrial service. To implement an antenna that receives vertical polarized signals on a low-profile aperture, a zero-phase constant, a surface wave, or a small magnetic loop may be applied.
Among conventional antennas, a metamaterial ring antenna having a height of 6.8 mm (λ0/28) generates vertically polarized current distribution. In this antenna, two vertical vias become in-phase due to the zero insertion phase between them. Alternately, a surface wave antenna capable of receiving vertical polarized signals has been suggested. This antenna consists of a thin grounded dielectric slab and periodic patches, and is excited by a circular patch. Surface wave diffraction on the slab causes vertical polarization, and a thickness of the antenna is 3 mm (0.05λ0). In another antenna, vertical polarization is obtained using a small magnetic loop because the magnetic loop is equivalent to an electric dipole.
A low-profile antenna that has better performance than the above-mentioned conventional antennas and can receive vertical polarized signals without affecting the appearance of a vehicle needs to be developed.